W29C020 Datasheet, PDF (3/21 Pages) Winbond – 256K X 8 CMOS FLASH MEMORY

W29C020
FUNCTIONAL DESCRIPTION
Read Mode
The read operation of the W29C020 is controlled by CE and OE , both of which have to be low for
the host to obtain data from the outputs. CE is used for device selection. When CE is high, the chip
is de-selected and only standby power will be consumed. OE is the output control and is used to gate
data from the output pins. The data bus is in high impedance state when either CE or OE is high.
Refer to the read cycle timing waveforms for further details.
Page Write Mode
The W29C020 is written (erased/programmed) on a page basis. Every page contains 128 bytes of
data. If a byte of data within a page is to be changed, data for the entire page must be loaded into the
device. Any byte that is not loaded will be erased to "FF hex" during the write operation of the page.
The write operation is initiated by forcing CE and WE low and OE high. The write procedure
consists of two steps. Step 1 is the byte-load cycle, in which the host writes to the page buffer of the
device.
Step 2 is an internal write (erase/program) cycle, during which the data in the page buffers are
simultaneously written into the memory array for non-volatile storage.
During the byte-load cycle, the addresses are latched by the falling edge of either CE or WE ,
whichever occurs last. The data are latched by the rising edge of either CE or WE , whichever
occurs first. If the host loads a second byte into the page buffer within a byte-load cycle time (TBLC) of
200 µS after the initial byte-load cycle, the W29C020 will stay in the page load cycle. Additional bytes
can then be loaded consecutively. The page load cycle will be terminated and the internal write
(erase/program) cycle will start if no additional byte is loaded into the page buffer A7 to A17 specify
the page address. All bytes that are loaded into the page buffer must have the same page address.
A0 to A6 specify the byte address within the page. The bytes may be loaded in any order; sequential
loading is not required.
In the internal write cycle, all data in the page buffers, i.e., 128 bytes of data, are written
simultaneously into the memory array. Before the completion of the internal write cycle, the host is
free to perform other tasks such as fetching data from other locations in the system to prepare to
write the next page.
Software-protected Data Write
The device provides a JEDEC-approved optional software-protected data write. Once this scheme is
enabled, any write operation requires a three-byte command sequence (with specific data to a
specific address) to be performed before the data load operation. The three-byte load command
sequence begins the page load cycle, without which the write operation will not be activated. This
write scheme provides optimal protection against inadvertent write cycles, such as cycles triggered by
noise during system power-up and power-down.
The W29C020 is shipped with the software data protection enabled. To enable the software data
protection scheme, perform the three-byte command cycle at the beginning of a page load cycle. The
device will then enter the software data protection mode, and any subsequent write operation must be
preceded by the three-byte command sequence cycle. Once enabled, the software data protection
Publication Release Date: February 1998
-3-
Revision A3